and these researches also seemed to point to a variety of creative
periods. In each period the earth's crust, composed of the various
strata, seemed to be differently constituted, just like the population
of animals and plants that then lived on it. Cuvier combined this
notion with the results of his own paleontological and zoological
research; and in his effort to get a consistent view of the whole
process of the earth's history he came to form the theory which is
known as "the catastrophic theory," or the theory of terrestrial
revolutions. According to this theory, there have been a series of
mighty cataclysms on the earth, and these have suddenly destroyed the
whole animal and plant population then living on it; after each
cataclysm there was a fresh creation of living things throughout the
earth. As this creation could not be explained by natural laws, it was
necessary to appeal to an intervention on the part of the Creator.
This catastrophic theory, which Cuvier described in a special work,
was soon generally accepted, and retained its position in biology for
half a century.

However, Cuvier's theory was completely overthrown sixty years ago by
the geologists, led by Charles Lyell, the most distinguished worker in
this field of science. Lyell proved in his famous Principles of
Geology (1830) that the theory was false, in so far as it concerned
the crust of the earth; that it was totally unnecessary to bring in
supernatural agencies or general catastrophes in order to explain the
structure and formation of the mountains; and that we can explain them
by the familiar agencies which are at work to-day in altering and
reconstructing the surface of the earth. These causes are--the action
of the atmosphere and water in its various forms (snow, ice, fog,
rain, the wear of the river, and the stormy ocean), and the volcanic
action which is exerted by the molten central mass. Lyell convincingly